cxenstored: fix systemd socket activation

cxenstored: fix systemd socket activation

There were two problems with original code:

1. sd_booted() was used to determined if the process was started by
   systemd, which was wrong.
2. Exit with error if pidfile was specified, which was too harsh.

These two combined made cxenstored unable to start by hand if it ran
on a system which had systemd.

Fix issues with following changes:

1. Use sd_listen_fds to determine if the process is started by systemd.
2. Don't exit if pidfile is specified.

Rename function and restructure code to make things clearer.

A side effect of this patch is that gcc 4.8 with -Wmaybe-uninitialized
in non-debug build spits out spurious warning about sock and ro_sock
might be uninitialized. Since CentOS 7 ships gcc 4.8, we need to work
around that by setting sock and ro_sock to NULL at the beginning of
main.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>